Handle meshlink_channel_close() being called in callbacks.

Handle meshlink_channel_close() being called in callbacks.

When it's called in a callback, we can't free the channel until the
function that called the callback has a chance to safely complete. This
is not a problem for regular receive and poll callbacks, but it is for AIO,
where there can be multiple outstanding AIO buffers that each need their
callback called to signal completion, and each of them could potentially
call meshlink_channel_close().

This also ensures that when the channel is explicitly closed by the
application, it will not receive any further callbacks.